Re: floating-point abuse in 2.1.113

Andi Kleen (ak@muc.de)
Thu, 6 Aug 1998 15:00:52 +0200

On Thu, Aug 06, 1998 at 02:49:39PM +0200, Richard B. Johnson wrote:
> On Thu, 6 Aug 1998, Andi Kleen wrote:
>
> > On Thu, Aug 06, 1998 at 03:36:48AM +0200, H. Peter Anvin wrote:
> > > > Gcc combines floating point constants in constant expressions, even when
> > > > cross compiling.
> > >
> > > Are you sure? Or only if the source and target use the same formats?
> >
> > Yes. read gcc/fold-const.c in the gcc source. They go into great pains
> > to do that.
> >
> > -Andi
> >
>
> Hmmmm. But gcc didn't do it in the kernel when it encountered some
> real number constants, cast to an int.

tfp.c:

int f()
{
return (int) (1.02 * 2.0 - 3.121232);
}

% gcc -v
Reading specs from /usr/lib/gcc-lib/i386-redhat-linux/2.7.2.3/specs
gcc version 2.7.2.3
% gcc -O2 -S tfp

tfp.s:

.file "tfp.c"
.version "01.01"
gcc2_compiled.:
.text
.align 4
.globl f
.type f,@function
f:
pushl %ebp
movl %esp,%ebp
movl $-1,%eax <------------------------
leave
ret
.Lfe1:
.size f,.Lfe1-f
.ident "GCC: (GNU) 2.7.2.3"

-Andi

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